1. Background of the Invention
In its more general aspect, the present invention relates to a high transverse-curvature tire for two-wheeled vehicles.
More particularly, the invention relates to a tire preferably, though not exclusively, usable in the so-called high-performance xe2x80x9ctouringxe2x80x9d motor-bikes having a high weight.
2. Known Art
In the field of production of tires for two-wheeled vehicles, and in particular, those intended for equipping the so-called xe2x80x9ctouringxe2x80x9d motor-bikes generally having a swept volume exceeding 1000 cm3, a high weight and a high torque, the need is increasingly felt of providing ever higher performances in terms of road behavior both in straight stretch and in curve, stability of the vehicle at high speed, kilometric yield, wear uniformity and regularity, and low weight.
In order to satisfy this need, tires for two-wheeled vehicles have been manufactured for a long time with a carcass structure comprising a couple of plies made of rubberized fabric reinforced with cords symmetrically inclined with respect to the equatorial plane of the tirexe2x80x94usually known as cross-plies carcassxe2x80x94and possibly an intermediate structure (breaker), also realized with couples of strips made of rubberized fabric provided with cords arranged at an angle with respect to the equatorial plane of the tire.
Even though this tire structure could ensure an extremely regular curve behavior of the motor-bike, the use of this type of tires involved problems of comfort, stability of the vehicle at high speed, and especially of irregular wear of the tread, often associated to a removal of bulk portions of rubber, according to a phenomenon known in the art by the term of: xe2x80x9cchunkingxe2x80x9d.
To try to prevent these drawbacks, it was then suggested to use a tire comprising a carcass structure of the so-called xe2x80x9cradialxe2x80x9d type, i.e. a tire provided with reinforcing cords essentially perpendicular to the equatorial plane of the tire, on which structure a belt structure comprising at least one couple of strips made of rubberized fabric provided with cords placed at an angle with respect to the equatorial plane of the tire or, alternatively, a cord winding, preferably made of metal, oriented in a circumferential direction and known in the art by the term of: zero-degree cords, is coaxially associated.
Even though the tires provided with a radial carcass have improved the situation in terms of comfort and vehicle straight-away stability at high speed with a substantial elimination of chunking phenomenaxe2x80x94to such an extent that they are considered top range products by those skilled in the artxe2x80x94they are not suitable for use in the so-called xe2x80x9ctouringxe2x80x9d motor-bikes having a high-weight and a high performance.
Actually, the motor-cycles of this type equipped with a tire having a radial carcass are particularly subject to an undesired xe2x80x9cfloatationxe2x80x9d effect while running along a curve, also known by the term of xe2x80x9cpudding effectxe2x80x9d, which causes the motor-bike to undergo a swaying phenomenon that would not be easily damped and that, in some instances, may be even self-amplifying. In extreme conditions, this phenomenon may even lead drivers to lose control of the vehicle, with obviously deleterious consequences.
According to the present invention, the Applicant has now understood that the problem of achieving adequate performances in terms of road behavior, both in straight stretches and along curves, stability of the vehicle at high speeds, kilometric yield, wear regularity, and low weight can be solved by combining a carcass structure having at least one ply provided with reinforcing cords inclined with respect to the equatorial plane of the tire, with a belt structure including a radially outer layer comprising a plurality of zero-degree cords and a radially inner layer comprising at least one sheet made of an elastomeric material.
In other words, the Applicant has become aware of the fact that the aforesaid problem can be effectively solved by renouncing to use the radial carcass, i.e., the very structure that is unanimously considered by those skilled in the art as the xe2x80x9ctopxe2x80x9d of the available technology, provided that the carcass equipped with reinforcing inclined cords is adequately separated from the belt layer comprising the zero-degree cords, by interposing a sheet made of an elastomeric material.
Hence, the present invention provides a tire comprising:
a) a carcass structure including at least one ply provided with reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the tire;
b) a belt structure coaxially extending around the carcass structure, including:
i) a radially inner layer comprising at least one sheet made of an elastomeric material;
ii) a radially outer layer including a plurality of circumferential coils, axially arranged side by side, of at least one inextensible cord, circumferentially wound at a substantially null angle with respect to the equatorial plane of the tire;
c) a tread, coaxially extending around the belt structure.
According to the invention, the Applicant has in particular and unexpectedly found that by interposing a sheet made of an elastomeric material between the carcass structure and the radially outer layer provided with zero-degree cords of the belt structure, it is possible to achieve at the same time both the desired features of road holding and curve stability imparted to the tire by the carcass structure, and the desired features of comfort, wear regularity and uniformity and stability of the vehicle at high speeds in straight stretches, imparted to the tire by the zero-degree cord layer of the belt structure.
More particularly, the Applicant has unexpectedly found that said sheet made of an elastomeric material allows the carcass structure and the zero-degree cords of the belt structure to exercise the aforesaid effects substantially independently from one another, preventing at the same time the zero-degree cords from damaging the reinforcing cords of the underlying carcass structure.
Thanks to the structural features of the carcass structure, the tire of the invention is able to develop high camber thrusts quite sufficient to balance the centrifugal thrust affecting the motor-bike, with a very homogeneous curve behavior.
According to the invention, it has also been found that camber and drift thrusts developed by the tire are increased by the combination of one or more carcass plies provided with reinforcing cords crown-forming a suitable angle with respect to the equatorial plane of the tire, with the radially outer layer incorporating the zero-degree cords of the belt.
Thanks to this feature, it was observed that a motor-bike equipped with a couple of tires having such a structure shows advantageous features of neutral driving, so that the driver can travel the curvilinear trajectory by simply inclining the vehicle and without correcting its trim, in particular the steering angle of the handlebar.
Besides, it was found that the cross-plies carcass lends the tire advantageous features of transverse strength and curve stability, while the aforesaid belt structure lends the tire dimensional stability, directional stability and lower energy absorption, achieving at the same time a low rolling resistance and a substantial disappearance of the chunking phenomenon.
In particular, it was observed that the presence in the belt structure of a radially outer layer provided with zero-degree cords allows to increase both the directional stability and the tire ground-contacting area, i.e. the area of contact with the ground, in all of the operating conditions of the tire.
Thanks to this feature, both the stresses due to scraping on the road and the stresses due to hysteresis dissipation in the rubber composition of the tread are reduced, with a substantial reduction in tire overheating due to the centrifugal force which develops during running, counter-balanced by the substantial inextensibility of the radially outer layer provided with zero-degree cords of the belt structure.
The adoption of such a combination of carcass and belt structures contributes to reduce the weight of the tire, with all of the ensuing advantages in terms of inertia due to the nonsuspended masses.
Preferably, the reinforcing cords incorporated in the ply(plies) of the carcass structure are essentially constituted by high modulus fibers of a material selected from the group comprising: rayon, nylon, polyethylene naphthalene 2,6 dicarboxylate (PEN) and polyethylene terephthalate (PET).
Preferably, the reinforcing cords incorporated in the ply(plies) of the carcass structure form a crown anglexe2x80x94measured at the equatorial plane of the tirexe2x80x94comprised between 25xc2x0 and 70xc2x0 with respect to the equatorial plane of the tire.
In this way an optimum curve behavior of the tire was observed, with the development of an adequate camber-thrust.
According to a particularly preferred embodiment of the invention, the carcass structure of the tire comprises at least one couple of superposed plies, each provided with reinforcing cords parallel to one another and oriented according to a direction inclined and opposite, preferably symmetrically, with respect to the equatorial plane of the tire with respect to the cords of the adjacent ply.
Preferably, the reinforcing cords incorporated in the carcass ply(plies) are made of a material different from the material of the reinforcing cords of the radially adjacent ply.
In such case, it is preferable to select materials having a modulus that increases as one radially proceeds toward the outside; in so doing, a further advantageous increase was observed in the effect of containing the thrusts due to the centrifugal force carried out by the belt structure.
Preferably, as the crown angle formed by the reinforcing cords of the carcass plies decreases, it may be convenient to insert between the latter at least one sheet made of an elastomeric material, possibly incorporating binding means suitable to increase its stretcheability properties without substantially altering the adhesion properties of the elastomeric material in the green state.
In this way, it is possible to absorb the tearing stresses that are generated between the carcass plies during rolling of the tire.
Preferably, the aforesaid binding means comprise the so-called aramid pulp (short fibrillated fibers of poly-paraphenylene-terephtalamide), of the type commercially known as xe2x80x9cKevlar(copyright)-pulpxe2x80x9d or xe2x80x9cTwaron(copyright)-pulpxe2x80x9d (Kevlar and Twaron are registered trademarks of DuPont and Akzo, respectively).
Preferably, said short fibrillated fibers are incorporated in the elastomeric material that makes up said sheet, in an amount comprised between 1 and 10 parts by weight per each 100 parts by weight of rubber (phr), and have a length comprised between 0.1 mm and 2.5 mm.
Advantageously, said sheet made of an elastomeric material has a thickness comprised between 0.075 mm and 0.5 mm.
According to the invention, the belt structure coaxially extending around the carcass structure comprises a radially inner layer including at least one sheet made of an elastomeric material.
As illustrated hereinabove, such sheet acts as an element suitable to separate, from both the structural and the functional viewpoints, the carcass structure from the layer including the zero-degree cords of the belt structure, preventing, among other things, the latter from damaging the reinforcing cords of the underlying carcass structure possibly causing a fatigue break of the same.
In order to optimize such separation action, the aforesaid sheet made of an elastomeric material has a thickness comprised between 0.5 mm and 3 mm and, still more preferably, between 1 mm and 3 mm.
Preferably, furthermore, such sheet made of an elastomeric material incorporates appropriate binding means suitable to increase its stretcheability properties without substantially altering the adhesion properties of the elastomeric material in the green state.
Preferably, the aforesaid binding means comprise the so-called aramid pulp (short fibrillated fibers of poly-paraphenylene-terephtalamide), of the type commercially known as xe2x80x9cKevlar(copyright)-pulpxe2x80x9d or xe2x80x9cTwaron(copyright)-pulpxe2x80x9d (Kevlar and Twaron are registered trademarks of DuPont and Akzo, respectively).
Preferably, said short fibrillated fibers are incorporated in the elastomeric material that makes up said sheet, in an amount comprised between 1 phr and 10 phr, and have a length comprised between 0.1 mm and 2.5 mm.
Preferably, said short fibrillated fibers are preoriented, for instance by means of a calendering operation, according to the main direction of the forces which the supporting element is subject to during the tire manufacturing process. Such direction is usually the circumferential direction of the tire and said preorientation operation is preferably carried out by calendering said sheet during its manufacture.
The elastomeric material reinforced with said aramid pulp has, at the green state, a tensile stress at break comprised between 3 MPa and 7 MPa, with a 50% elongation at a tensile stress comprised between 0.6 MPa and 3 MPa, while the same elastomeric material without aramid pulp has, at the green state, a tensile stress at break comprised between 1 MPa and 2 MPa, with a 50% elongation at tensile stress comprised between 0.2 MPa and 0.5 MPa.
Preferably, the material that constitutes the sheet of the radially inner layer of the belt structure is a natural rubber-based composition containing carbon-black in an amount comprised between 30 phr and 70 phr, filled with the usual ingredients known in the art (plasticizers, protecting agents, antidegradation agents, vulcanizers), so as to obtain an elastomeric matrix as similar as possible to that of the elements to which said sheet has to adhere.
As illustrated hereinabove, the radially outer layer of the belt structure comprises a plurality of circumferential coils, axially arranged side by side, of at least one inextensible cord, wound at a substantially null angle with respect to the equatorial plane of the tire, and usually known in the art by the term of zero-degree cords.
Preferably, the zero-degree cord or cords of the radially outer layer of the belt structure are high-elongation metal cords with high carbon content steel wires.
Alternatively, the zero-degree cords may be textile cords made of aramid.
When a tire to be mounted on the rear wheel of a motor-bike is to be manufactured, the cord coils arranged at a substantially null angle with respect to the equatorial plane of the tire are preferably distributed with a constant density throughout the axial development of the belt structure.
Instead, when a tire to be mounted on the front wheel of a motor-bike is to be manufactured, the ford coils arranged at a substantially null angle with respect to the equatorial plane of the tire are preferably distributed with a variable density throughout the axial development of the belt structure.
According lo this last embodiment of the invention, the distribution density of the cord coils progressively changes along the layer, from the equatorial plane toward the ends, preferably according to a prefixed relation, having a value not higher than 15 cords/cm in an area placed on either side of the equatorial plane of the tire.
In this way, it is advantageously possible to obtain a belt structure which is at the same time flexible in the middle, to absorb and damp the vibrations due to the ground roughness, and rigid along the sides, to develop high slip thrusts.
According to the experiments carried out by the Applicant, such relation may conveniently have the following expression:       N    ⁢          xe2x80x83        ⁢    x    =      K    ⁢          xe2x80x83        ⁢                  R        2                    r        2              ⁢    N    ⁢          xe2x80x83        ⁢    o  
wherein:
No is the number of cord coils arranged in a central portion of unitary length located on either side of the equatorial plane of the belt structure;
R is the distance between the center of said portion and the rotation axis of the tire;
r is the distance between the center of the generic unitary portion comprised between the equatorial plane and the ends of said radially outer layer of the belt structure and the rotation axis of the tire;
K is a parameter that takes into account the constituent material and the formation of cords, as well as the amount of rubber around the cord, and the weight of a portion of the radially inner layer of the belt structure at said unitary portion, which is variable with variations in the material type and in the structural features of the radially inner layer of the belt structure along the crown profile that diverge from the reference values.
This parameter K may have a value of substantially close to 1 if all the cords have the same formation and all the connected materials are the same throughout the layers, or different values according to the variations in the materials and formation of the reinforcing elements along the peripheral development of the belt structure.
A distribution of the cords in accordance with such relation ensures both uniformity of the stress acting on the belt structure during use of the tire as a consequence of the centrifugal force applied, and the necessary differentiated stiffness along the axial direction.
Obviously, those skilled in the art may find other relations which, depending on the aforesaid design variables, would allow to obtain at the same time a differentiated stiffness along the axial direction and a stress uniformity in the belt structure of the running tire, by varying in a controlled and predetermined manner the density of the aforesaid cords.
As to the density of the zero-degree cords in the portion located on either side of the equatorial plane, where the maximum thinning out take place, this density, for a tire to be mounted on the front wheel, is preferably not greater than 8 cords/cm and more preferably comprised between 3 cords/cm and 6 cords/cm.
For a tire to be mounted on the front wheel, the width of said portion varies preferably from 10% to 30% of the axial development of the belt.
Conveniently, for a tire to be mounted on the front wheel, the quantity of cords in said central portion is equal to a value comprised between 60% and 80% of the cords quantity near the shoulders of the tire, where the density of said cords is preferably not greater than 10 cords/cm and more preferably included between 6 cords/cm and 8 cords/cm.